Scientists have offered two different explanations associated to the decline in the growth of methane in the atmosphere in the last decade of the 20th century, before it rose again.

Two papers that are published in today's issue of the journal Nature, present two very different theories related to the possible cause of the decline in Methane, but both the researches agree that human activities are the key element.

The first theory suggests that the decline was caused by greater commercial use of natural gas. The study was led by Dr Murat Aydin from the University of California, Irvine.

"We went after ethane - it's another hydrocarbon similar to methane, it has common sources, but is easier to trace. We determined what ethane did during the second half of the 20th century using ancient air that we collected at polar ice sheets,” said Dr Murat.

The second links increased use of artificial fertilizer in Asia as the cause of the decline. This study was conducted by researchers from the same department in the same university.

"Approximately half of the decrease in methane can be explained by reduced emissions from rice agriculture in Asia associated with increases in fertiliser application and reductions in water use," says the lead author Dr Fuu Ming Kai.

Methane is believed to the most potent greenhouse gases, which taps over 20 times more atmospheric heat than carbon dioxide. The level of Methane has been increasing from a wide variety of sources, including energy production, the burning of forests, and increased numbers of cattle and sheep.

However, the level decreased between 1980 and the new century, which left scientists confused over the possible causes.